Main beam alignment verification for tracking antennas

ABSTRACT

A method for main beam alignment verification includes providing data pertaining to one or more patterns associated with an antenna, measuring power levels of a signal acquired by the antenna, and comparing the measured power levels with the data to determine whether a direction of the signal is incident upon a main beam of the antenna.

TECHNICAL FIELD

The invention relates generally to antenna tracking and, in particular,to main beam alignment verification for angle tracking antennas.

BACKGROUND ART

Tracking antenna systems dynamically follow changes in a direction of areceived signal, and position a tracking antenna to align the signalwith the peak level of the main beam of the tracking antenna. Suchsignal alignment results in coincidence of the signal and a portion ofthe main beam that provides maximum antenna gain and thus systemsensitivity.

Various approaches to antenna tracking involve aligning the main beampeak of an antenna with the signal by using open commanding and relativepower measurements. One example, referred to as “step track,” involvespositioning an antenna in a nominal direction, and commanding theantenna in equal but opposite angular offsets and measuring receivedsignal power at each offset position. If the received signal powerlevels are equal, the antenna is correctly aligned. If the receivedpower levels are unequal, the difference in the power levels can be usedto correct the antenna alignment. The process is repeated in theorthogonal plane. The step track technique is periodically repeated tovalidate correct antenna alignment and to follow any changes in thedirection of the signal.

Other approaches to antenna tracking involve a closed loop techniquereferred to as “monopulse.” Two types of antenna patterns are used insuch techniques. The first type of pattern has a maximum gain value thatis coincident with the axis of the antenna, and is used for datareception. The second type of pattern has a null on the axis of theantenna and, to first order, has a linear variation with displacementsfrom the axis and typically a phase difference between the data patternand the tracking pattern that coincides with the azimuth angle of thesignal direction. This behavior, the linear increase with deviation fromaxis and the phase difference, is used by an antenna control unit as anerror signal, thereby permitting implementation of a closed looptracking system that dynamically follows changes in the direction of thesignal.

Unfortunately, these angle-tracking techniques depend upon initialantenna pointing (prior to initiation of antenna tracking) to align thesignal within the main beam angular extent of the antenna. In somecases, however, such alignment is not assured.

One prior method for verifying this alignment involves using a smallerguard antenna together with the larger main antenna that is used fordata reception. In practice, the smaller antenna is about 1/10 thediameter of the main antenna to obtain the required gain and patterncharacteristics to envelope the sidelobes of the main antenna. Thesignal levels received by the main antenna and the guard antenna arethen compared. If the signal level of the main antenna exceeds thesignal level of the guard antenna, the antenna is aligned within themain beam where the main antenna gain is higher than the gain of theguard antenna. If the signal level of the main antenna is comparable orless than the signal level of the guard antenna, then the signal isaligned with sidelobes of the main antenna.

In addition to requiring a second antenna, another short-fall of thistechnique is that the boresight of the guard antenna needs to bemaintained coincident with the main antenna. Moreover, the smaller guardantenna needs to be mechanically isolated from the main antenna to avoiddeforming the main antenna and its patterns, and the mechanical balanceof the assembly needs to be maintained. When the received signal levelfluctuates, as commonly occurs with multipath at low elevation angles,the guard antenna requires a second tracking receiver so that the signallevels in the guard and main antennas can be simultaneously measured.Aside from the expense of an additional tracking receiver, the tworeceivers need to be reliably calibrated so that the same received powerlevel results in the same indicated signal levels. This calibration isneeded so that the signal level comparison can be used to reliablyverify main beam alignment. Additionally, for large antennas thatrequire a protective radome to avoid pointing errors caused by windloading, a larger radome that envelops both the main and guard antennasis significantly more expensive than a radome for the main antennaalone.

Thus, it would be useful to be able to provide a main beam alignmentverification alternative to the prior approaches. It would also bedesirable to be able to provide a cost effective method for verifyingthe main beam alignment of received signals. It would also be useful tobe able to provide a mechanism for verifying main beam alignment withoutimposing additional hardware capabilities, i.e., using existing antennahardware.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot showing a typical antenna pattern for a data channel;

FIG. 2 is a plot showing examples of data (sum) and tracking(difference) patterns;

FIG. 3 is a plot of an example monopulse error response;

FIG. 4 illustrates an example of an antenna tracking system and signalsarriving at different angles relative to a main beam and a side lobe,which are shown in cross-section;

FIG. 5 illustrates an example open loop antenna tracking system;

FIG. 6 illustrates an example closed loop antenna tracking system;

FIG. 7 is a flowchart of an example method for open loop antennatracking; and

FIG. 8 is a flowchart of an example method for closed loop antennatracking.

DISCLOSURE OF INVENTION

Various methods for verifying main beam alignment according to thepresent invention generally involve measuring the width of the lobecontaining the received signal. As shown in FIG. 1, in a typical antennapattern 100 for a data channel, the angular width of the main beam 102of the antenna is roughly twice the angular width of the sidelobes 104of the antenna. In an example embodiment, open loop commanding is usedto measure the width of the lobe containing the received signal andprovides a mechanism for verifying main beam alignment. By way ofexample, such an embodiment can be used with antenna tracking methodsthat utilize open loop techniques.

As discussed below, various embodiments can be implemented with existingantenna hardware operated in conjunction with appropriate softwarecontrols and received power measurements. Furthermore, the methodsdescribed herein do not require a guard antenna and, therefore, do notrequire the additional aperture needed for the guard antenna or anadditional tracking receiver to measure the signal levels received bythe guard antenna.

FIG. 4 illustrates an example of an antenna tracking system 400 andsignals arriving at different angles relative to a main beam 402 and asidelobe 404. The antenna tracking system 400 include an antenna 406(e.g., a reflector antenna), an antenna positioner 408 and data andtracking receivers 410 as shown. In this example, an arrow 412 indicatesa signal arrival direction incident upon the main lobe 402 as shown. Anarrow 414 indicates a signal arrival direction incident upon thesidelobe 404 as shown.

Referring to FIG. 5, an example open loop antenna tracking system 500exploits the circumstance that the angular width of the main beam isroughly twice that of the sidelobes. In this embodiment, the differencein the angular widths of the main beam and the sidelobes is used toverify main beam alignment.

The example open loop antenna tracking system 500 includes an antenna502, a positioner 504, a data receiver 506 and an antenna control system508, configured as shown. The antenna control system 508 is providedwith a received signal power input from the data receiver 506. In thisexample embodiment, satellite ephemeris and antenna data inputs are alsoprovided to the antenna control system 508.

In operation, after the data receiver 506 acquires the signal, a signallevel measurement is made. A typical received signal level indication isthe automatic gain control (AGC) voltage of the receiver. The antenna502 is then offset in equal angular increments in opposite directionsand the received power levels at each offset position are measured usinga step track procedure, for example. In this example embodiment, themeasured power levels are used for an additional purpose. The threemeasured power levels, at the initial acquisition point and the twoangularly displaced positions, are then compared to the a priori knownmain beam shape and the sidelobe shape of the antenna 502. The a prioriantenna data and the measured power levels are processed by the antennacontrol system 508 which includes, for example, a main beam searchalgorithm in firmware and step track decision logic for sidelobedetermination. Because the angular width of the sidelobes is less thanthat of the main beam, the power level changes at the angular offsetpositions are more drastic than those at the main beam. By thiscomparison of the three power levels with the a priori antenna patterndata, decisions are made as to main beam alignment or sidelobealignment. In this example embodiment, the process is then repeated inan orthogonal direction.

FIG. 7 shows an example open loop antenna tracking method 700. At step702, a determination is made as to whether the receiver has acquired asignal. If the determination is negative, at step 704, the antenna isrepositioned and acquisition reattempted until successful. At step 706,step track is initiated to make the power measurements as discussedabove. At step 708, the measured power levels are compared to the apriori antenna data and a determination is made as to whether a receivedsignal appears to be incident upon the main beam. If this determinationis negative, then at step 710 the antenna is aligned with the sidelobepeak. Once aligned with the sidelobe peak, at step 712 the antenna isstepped to the next sidelobe peak using an open loop command. In thisexample embodiment, the a priori antenna pattern is used for thisprocess, and typically the antenna step size is about one and one-halfbeamwidths between sidelobe peak positions. At step 714, a determinationis made as to whether the received signal increases in strength and step712 is repeated until this determination is affirmative. If the openloop repositioning command encounters the main beam of the antenna, theantenna will not be aligned with the main beam peak but will besufficiently high on the main beam to provide a clear indication of mainbeam alignment. At step 716, the received signal is aligned withboresight. At step 718, this alignment is re-verified using, forexample, either a step track or a monopulse monitoring technique.

In an example embodiment, open loop commanding techniques are used tofind the main beam. By using an open loop command to the next sidelobepeak and/or the main antenna beam, there may be sufficient receivedsignal power to maintain receiver acquisition, and therefore it will notbe necessary to move the antenna and try to reacquire the receiver. Ifthe tracking receiver has not acquired, a two-dimensional raster scan(e.g., in azimuth and elevation) can be used. In such a case, once thetracking receiver has acquired, the open loop commanding for antennarepositioning can be used.

Other information can be used in the above search process. For example,in some cases, an estimate of the nominal signal level may be available.Also by way of example, when an acquisition at low elevation angles isbeing performed, the search for the main beam can exclude those antennapositions where the main beam would be positioned below the horizon.

Signal level fluctuations can have an impact on open loop antennatracking techniques. Multipath is a common cause of such fluctuationsthat occurs at low elevation angles. For step track, the fluctuationscan be reduced by dwelling at each angular position for a sufficientamount of time to average the power measurements. Averaging at lowelevation angles may have limited effectiveness in cases where themultipath has a strong specular component. Averaging, particularly inthe elevation coordinate, is less effective since the average of thedirect signal and the multipath component has a non-zero mean value. Itis generally more effective to average in the azimuth coordinate. Analternative approach is to wait until the satellite has increased in itselevation angle and the spatial filtering of the pattern of the receiveantenna reduces multipath. When the satellite is higher above thehorizon, a further search for the main beam can be conducted at theanticipated position of the satellite at that time. The main beamacquisition verification method described above can be used for openloop tracking designs using step track techniques or with closed loopmonopulse-based tracking designs.

In an example embodiment, a method for main beam alignment verificationincludes providing data including main beam and sidelobe angular widthsfor an antenna, measuring power levels of a signal acquired by theantenna at multiple antenna positions, and comparing the measured powerlevels with the data to determine whether a direction of the signal isincident upon a main beam of the antenna.

In another example embodiment, a method for main beam alignmentverification includes providing data pertaining to one or more patternsassociated with an antenna, measuring power levels of a signal acquiredby the antenna, and comparing the measured power levels with the data todetermine whether a direction of the signal is incident upon a main beamof the antenna.

Various methods for verifying main beam alignment according to thepresent invention involve examining monopulse error response. Referringto FIG. 6, an example closed loop antenna tracking system 600 exploits amonopulse-based design and includes an antenna 602, a positioner 604, adata receiver 606, a tracking receiver 608 and an antenna control system610, configured as shown. In an example embodiment, a monopulse errorresponse is examined and serves as a basis to decide whether the antennais aligned within the main beam. By way of example, such an embodimentcan be used to verify main beam alignment prior to initiating a closedloop tracking operation. Also by way of example, such an embodiment canbe used with antenna tracking methods that utilize monopulse-trackingtechniques.

FIG. 2 is a plot showing examples of a data (sum) pattern 200 and atracking (difference) pattern 202 (in dashed lines) for a typicalantenna. In this example, the main beam peak of the data pattern 200 iscoincident with the null of the tracking pattern 202. Referring to FIG.3, over the same angular region, a monopulse error response 300 resultsfrom dividing the tracking pattern 202 by the data pattern 200. Theactual values depend on the difference in the path loss between the dataand tracking channel and the coupler coefficient.

In an example embodiment, a determination is made as to whether ameasured monopulse output is consistent with the main beam. By way ofexample, the monopulse error response is determined a priori bymeasurements and/or analyses of the tracking system for the antenna. Inan example embodiment, the monopulse error response is determined overthe linear error response of the main lobe (which for practical antennadesigns is typically somewhat less than the width of the main beam asshown in FIG. 3) and also out into the sidelobe regions; this extendedresponse provides a data base that is used to verify main beamalignment. A common implementation couples a small portion of thetracking signal onto the data signal. The tracking signal is switched toquadrature positions and when added to the data signal, produces anamplitude modulation indicating antenna misalignment that is acted on bythe antenna control system.

In an example main beam alignment method, a determination is made as towhether indicated tracking error behavior corresponds to variations thatresult in the main beam of the antenna. FIG. 8 shows an example closedloop antenna tracking method 800. At step 802, a determination is madeas to whether the receiver has acquired a signal. If the determinationis negative, at step 804, the antenna is repositioned and acquisitionreattempted until successful. As shown in FIG. 2, the data and trackingantenna patterns indicate regions within the sidelobe response of theantenna where the tracking pattern 202 can exceed the data pattern 200and overmodulation occurs. At step 806, a determination is made as towhether modulation values are within a range resulting from antennapositions within the main beam. If an overmodulation is present, at step808 the antenna is repositioned (e.g., one beamwidth) until a modulationcondition is detected. If the modulation level lies within a range thatis credible for the antenna, the antenna can be open loop commanded to aposition that corresponds to the boresight location for the main beam.In this example embodiment, at step 810 the monopulse error is measured,and at step 812 the position correction is commanded. At step 814, ifthe open loop repositioning results in the expected reduction of themodulation from the tracking channel consistent with the a prioriantenna data for the main beam of the antenna, then the system doesindeed have alignment within the main beam. As shown in FIG. 3, theerror slope within the main beam region 302 is less than the error slopewhen a linear region exists within the sidelobe regions 304 of theantenna. If the determination at step 814 is negative, a sidelobe of theantenna is aligned with the signal. Then, at step 816 the a prioriantenna data is used to align the antenna to the sidelobe peak, and theantenna is stepped to the next sidelobe position until the results ofthe determination made in step 814 are consistent with main beampositioning. Then, at step 820 closed loop operation (e.g., autotrack)is initiated.

As discussed above, open loop pointing techniques can be used to realignthe antenna with the main beam. The monopulse error response can also beused to realign the antenna to its main beam of the data pattern. Aswith the open loop tracking technique, measurement and/or analyses canbe used to determine the monopulse error response and knowledge of thisresponse can be used in open loop commanding to determine therealignment. Linear regions of the monopulse error response alsocoincide with the sidelobe peaks, and open loop commanding can be usedto reposition the antenna to realign the antenna. Open loop commandingcan be used in conjunction with determining whether an error slopecorresponds to a value within the main beam or within the sidelobes. Ifthe system has a closed loop monopulse-based tracking system, acombination of the approaches described herein can be used to obtain afurther assurance of main beam acquisition.

In an example embodiment, a method for main beam alignment verificationincludes providing data including monopulse error responses for a mainbeam region and sidelobe regions of an antenna, measuring power levelsof a signal acquired by the antenna at multiple antenna positions, andcomparing the measured power levels with the data to determine whether adirection of the signal is incident upon a main beam of the antenna.

Although the present invention has been described in terms of theexample embodiments above, numerous modifications and/or additions tothe above-described embodiments would be readily apparent to one skilledin the art. It is intended that the scope of the present inventionextend to all such modifications and/or additions.

1. A method for main beam alignment verification, comprising: providingdata pertaining to one or more patterns associated with an antenna;measuring, at at least one data receiver, power levels of a signalacquired by the an antenna; and comparing, at at least one antennacontrol system, the measured power levels with the data pertaining toone or more patterns associated with the antenna; and todeterminedetermining, at the at least one antenna control system,whether a direction of the signal is incident upon a main beam of theantenna based on the comparison.
 2. The method for main beam alignmentverification of claim 1, wherein the one or more patterns include a datapattern.
 3. The method for main beam alignment verification of claim 1,wherein the one or more patterns include data and tracking patterns. 4.The method for main beam alignment verification of claim 1, wherein thedata includes main beam and sidelobe angular widths for the antenna. 5.The method for main beam alignment verification of claim 1, wherein thedata includes monopulse error responses for a main beam region andsidelobe regions of the antenna.
 6. The method for main beam alignmentverification of claim 1, wherein the data is a priori.
 7. The method formain beam alignment verification of claim 1, wherein multiple samples ofthe measured power levels at a common antenna position are averaged. 8.The method for main beam alignment verification of claim 1, whereinpower levels measured in the azimuth coordinate are averaged.
 9. Amethod for main beam alignment verification, comprising: providing dataincluding main beam and sidelobe angular widths for an antenna;measuring, at at least one data receiver, power levels of a signalacquired by the an antenna at multiple antenna positions; and comparing,at at least one antenna control system, the measured power levels withthe data pertaining to one or more patterns associated with the antenna;and to determinedetermining, at the at least one antenna control system,whether a direction of the signal is incident upon a main beam of theantenna based on the comparison.
 10. The method for main beam alignmentverification of claim 9, wherein the data is a priori.
 11. The methodfor main beam alignment verification of claim 9, wherein multiplesamples of the measured power levels at a common antenna position areaveraged.
 12. The method for main beam alignment verification of claim9, wherein power levels measured in the azimuth coordinate are averaged.13. The method for main beam alignment verification of claim 9, furtherincluding using the data to reposition the antenna.
 14. The method formain beam alignment verification of claim 9, further including using thedata to reposition the antenna after a determination has been made thatthe signal is not incident upon the main beam.
 15. The method for mainbeam alignment verification of claim 9, further including using the datato reposition the antenna to align the signal with a sidelobe of theantenna.
 16. A method for main beam alignment verification, comprising:providing data including monopulse error responses for a main beamregion and sidelobe regions of an antenna; measuring, at at least onedata receiver and at least one tracking receiver, power levels of asignal acquired by the an antenna at multiple antenna positions; andcomparing, at at least one antenna control system, the measured powerlevels with the data pertaining to one or more patterns associated withthe antenna and including monopulse error responses for a main beamregion and sidelobe regions of the antenna; and to determinedetermining,at the at least one antenna control system, whether a direction of thesignal is incident upon a main beam of the antenna based on thecomparison.
 17. The method for main beam alignment verification of claim16, wherein the monopulse error responses are determined from data andtracking patterns of the antenna.
 18. The method for main beam alignmentverification of claim 16, wherein the data is a priori.
 19. The methodfor main beam alignment verification of claim 16, further includingusing the data to reposition the antenna.
 20. The method for main beamalignment verification of claim 16, further including using the data toreposition the antenna after a determination has been made that thesignal is not incident upon the main beam.
 21. The method for main beamalignment verification of claim 16, further including using the data toreposition the antenna to align the signal with a sidelobe of theantenna.
 22. A system for determining main beam and sidelobe alignment,comprising: an antenna for acquiring a signal and outputting a receivedsignal; a positioner for rotating the antenna to offset positions; adata receiver for receiving the received signal and outputting data andpower of the received signal; and an antenna control system forcontrolling the positioner and for processing outputs of the datareceiver and a priori data, wherein the offset positions of the antennaare spaced apart in opposite directions and with equal angular widths,and the system is configured to measure received signal power uponinitially acquiring the signal and at each of the offset positions,compare the measured received signal powers with a priori antenna data,and determine main beam and sidelobe alignment based on the comparison.23. The system of claim 22, wherein the a priori antenna data is a mainbeam shape and a sidelobe shape.
 24. The system of claim 23, wherein thea priori antenna data further comprises satellite ephemeris data. 25.The system of claim 22, wherein the received signal power is measured ateach of the offset positions and when the signal is initially acquiredwith a step track procedure.
 26. The system of claim 25, wherein theantenna control system further comprises logic for the step trackprocedure.
 27. The system of claim 22, wherein an automatic gain controlsignal of the data receiver provides the received signal power.
 28. Asystem for determining main beam and sidelobe alignment comprising: anantenna for acquiring a signal and outputting a received sum signal andreceived difference signal; a positioner for rotating the antenna tooffset positions; a data receiver for outputting data and power of thereceived sum signal; a tracking receiver for tracking the receiveddifference signal and outputting power of the received differencesignal; and an antenna control system for controlling the positioner andfor processing outputs of the tracking receiver and a priori antennadata, wherein the offset positions of the antenna are spaced apart inopposite directions with equal angular widths and, received sum signalpower is measured upon initially acquiring the signal and at each of theoffset positions, the measured received sum signal powers are comparedwith the a priori antenna, and main beam and sidelobe alignment aredetermined based on the comparison.
 29. The system of claim 28, whereinthe a priori antenna data is monopulse error slope data over extendedangular range.
 30. The system of claim 28, wherein the received sumsignal power at each of the offset positions and upon acquiring thesignal is measured with a step track procedure.
 31. The system of claim30, wherein the antenna control system further comprises logic for thestep track procedure.
 32. A non-transitory computer-readable mediumcontaining executable instructions that, when executed by a machine,cause the machine to implement a method for main beam alignmentverification, comprising: receiving measured power levels of a signalacquired by an antenna; accessing data pertaining to one or morepatterns associated with the antenna; comparing the measured powerlevels with the data pertaining to one or more patterns associated withthe antenna; determining whether a direction of the signal is incidentupon a main beam of the antenna based on the comparison; and commandingthe antenna to be repositioned based on whether the direction of thesignal is incident upon the main beam of the antenna.
 33. Thecomputer-readable medium of claim 32, wherein the one or more patternsinclude a data pattern.
 34. The computer-readable medium of claim 32,wherein the one or more patterns include data and tracking patterns. 35.The computer-readable medium of claim 32, wherein the data includes mainbeam and sidelobe angular widths for the antenna.
 36. Thecomputer-readable medium of claim 32, wherein the data includesmonopulse error responses for a main beam region and sidelobe regions ofthe antenna.
 37. The computer-readable medium of claim 32, whereinexecution of the instructions further comprises averaging multiplesamples of the measured power levels at a common antenna position. 38.The computer-readable medium of claim 32, wherein execution of theinstructions further comprises averaging power levels measured in theazimuth coordinate.
 39. A non-transitory computer-readable mediumcontaining executable instructions that, when executed by a machine,cause the machine to implement a method for main beam alignmentverification, comprising: receiving measured power levels of a signalacquired by an antenna at multiple antenna positions; accessing datapertaining to one or more patterns associated with the antenna;comparing the measured power levels with the data pertaining to one ormore patterns associated with the antenna; determining whether adirection of the signal is incident upon a main beam of the antennabased on the comparison; and commanding the antenna to be repositionedbased on whether the direction of the signal is incident upon the mainbeam of the antenna.
 40. The computer-readable medium of claim 39,wherein execution of the instructions further comprises averaging ofmultiple samples of the measured power levels at a common antennaposition.
 41. The computer-readable medium of claim 39, whereinexecution of the instructions further comprises averaging of powerlevels measured in the azimuth coordinate.
 42. The computer-readablemedium of claim 39, wherein execution of the instructions furthercomprises generating a positioner signal to reposition the antenna. 43.The computer-readable medium of claim 39, wherein execution of theinstructions further includes generating a positioner signal toreposition the antenna after a determination has been made that thesignal is not incident upon the main beam.
 44. The computer-readablemedium of claim 39, wherein execution of the instructions furthercomprises generating a positioner signal to reposition the antenna andto align the signal with a sidelobe of the antenna.
 45. A non-transitorycomputer-readable medium containing executable instructions that, whenexecuted by a machine, cause the machine to implement a method for mainbeam alignment verification, comprising: receiving measured power levelsto form measured error responses at multiple antenna positions;accessing data pertaining to one or more patterns associated with theantenna and including monopulse error responses for a main beam regionand sidelobe regions of an antenna; comparing the measured errorresponses with the data pertaining to one or more patterns associatedwith the antenna and including monopulse error responses for a main beamregion and sidelobe regions of the antenna; determining whether adirection of the signal is incident upon a main beam of the antennabased on the comparison; and commanding the antenna to be repositionedbased on whether the direction of the signal is incident upon the mainbeam of the antenna.
 46. The computer-readable medium of claim 45,wherein execution of the instructions further comprises determining themonopulse error responses from data and tracking patterns of theantenna.
 47. The computer-readable medium of claim 45, wherein executionof the instructions further comprises generating a positioner signal toreposition the antenna.
 48. The computer-readable medium of claim 45,wherein execution of the instructions further comprises generating apositioner signal to reposition the antenna after a determination hasbeen made that the signal is not incident upon the main beam.
 49. Thecomputer-readable medium of claim 45, wherein execution of theinstructions further comprises generating a positioner signal toreposition the antenna and to align the signal with a sidelobe of theantenna.